Move the code that implements VF version compatibility on the PF side to
a separate function so that it can be reused when doing VM live
migration.

This does not introduce any functional change.
Signed-off-by: Xin Zeng <xin.zeng@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>